GUI bugfix (histogram) + touchup

- Fix the Histogram background and grid lines
- Add 1 pixel of empty space between in the Histogram panel
- Add a light grey color border for Gtk::DrawingArea oject, for those
using the render_frame method
- In the Toolbox, make the nested MyExpander and Gtk::Frame lighter, and
adjust the Gtk::DrawingArea background color accordingly
- All added class to the MyCurve has been removed, so that the
Gtk::Drawing object's style is used
- The Retinex Settings' expander is now level 2 for GUI consistency

rtgui/cropwindow.cc

@@ -1057,7 +1057,7 @@ void CropWindow::expose (Cairo::RefPtr<Cairo::Context> cr)
     options.bgcolor = backColor;
 
     if (backColor == 0) {
-        iarea->get_style_context()->render_background(cr, x + imgAreaX, y + imgAreaY, imgAreaW, imgAreaH);
+        style->render_background(cr, x + imgAreaX, y + imgAreaY, imgAreaW, imgAreaH);
     } else {
         if (backColor == 1) {
             cr->set_source_rgb (0, 0, 0);
@@ -1661,6 +1661,8 @@ void CropWindow::expose (Cairo::RefPtr<Cairo::Context> cr)
         }
     }
 
+    style->render_frame (cr, x + imgAreaX, y + imgAreaY, imgAreaW, imgAreaH);
+
     //t2.set ();
 //    printf ("etime --> %d, %d\n", t2.etime (t1), t4.etime (t3));
 }